1. Field of the Invention
This invention relates generally to the field of gas turbine engines for the generation of electricity and, more particularly, to a method for controlling gas turbine engine acceleration and exhaust gas temperatures after ignition but prior to reaching self-sustaining speed.
2. Description of the Prior Art
The starting of a gas turbine engine is a complex operation. Typically, before the gas turbine engine is run on its own power, the gas turbine engine must be accelerated by an external electric source, such as a battery, to provide sufficient airflow to a combustor via a compressor for ignition. In a turboalternator having a permanent magnet rotor/generator coupled to a gas turbine engine, supplying electrical power to the permanent magnet rotor/generator will have it function as a motor to drive the gas turbine engine. Typically, engine speed varies as a function of the torque versus speed characteristics of the starter motor.
Compressed air from the compressor to the combustor increases generally with gas turbine engine speed. Typically, the gas turbine rotor is directly coupled to the compressor rotor. Ignition occurs when the speed of the engine produces enough compressed air to produce the correct ratio of air with the fuel supplied.
After ignition occurs, but prior to the gas turbine engine reaching self-sustaining speed, the acceleration rate of the gas turbine engine increases rapidly, thereby increasing the airflow. The increase in airflow requires an increase in the flow of fuel to maintain the proper fuel-to air ratio. The increased and continuous combustion occurring in the combustor also creates very high exhaust gas temperatures (EGTs). In a prior art starting procedure, the rapid increase in acceleration and EGT is controlled by setting a maximum EGT and using an open-loop control system. Thus, the fuel valves are opened until the exhaust gas temperature reaches the maximum point. At the same time, the external source is also contributing energy to the engine. The prior art starting procedure wastes energy and creates wear on the gas turbine engine. A smoother acceleration rate and lower EGT helps to prevent wear and save energy.
It is, therefore, an object of the present invention to prevent wear and conserve energy on a gas turbine engine by providing a method for controlling the acceleration rate and EGTs of a gas turbine engine after ignition, but prior to the engine reaching self-sustaining speed.
The present invention provides the method for controlling the acceleration rate and EGT (exhaust gas temperature) of a gas turbine engine that utilizes a turbine engine compressor, an annular combustor, an external electric source, such as a battery, and a control system containing a PID (Proportional Integral Derivative) controller. The annular combustor can include a single fuel source or multiple fuel sources.
Prior to operating a gas turbine engine, certain variables should be defined. A moderate EGT can be defined to ensure that the combustion chamber will not overheat. A table of rates of change of EGT can also be defined. The parameters of the table can include: 1) rotational speed of the turbine drive shaft, i.e., rotational speed; 2) temperature of the air at the inlet to the engine compressor, i.e., inlet temperature; and 3) EGT. A table of acceleration rates can be defined with parameters that are the same as the above three variables. Finally, a maximum amount of time for the gas turbine engine to reach self-sustaining speed after ignition is determined based on testing and experimentation.
The method of operating a gas turbine engine after ignition but prior to reaching self-sustaining speed involves several steps: (1) monitoring EGT, the rate of change of EGT, rotational speed of the engine rotor or turbine drive shaft, the acceleration of the engine rotor or turbine drive shaft, and the inlet temperature; (2) requesting a rate of change of EGT from a table; (3) enabling a PID controller to determine the position of the fuel valve based upon the requested rate of change of EGT; (4) retrieving two acceleration rates: an acceleration rate from the table and a current acceleration rate; (5) operating the external electric source using the greater acceleration rate; and (6) exiting the control loop either upon the gas turbine engine reaching self-sustaining speed or upon the gas turbine engine not reaching self-sustaining speed within the predetermined time.